The World Health Organization has determined that anthropometry, the comparative measurement and study of the human body, reflects both nutritional and health status and that it is predictive of performance, health, survival, and the risk of disease. Anthropometric measurements can also be used to assess the need of an individual for special services such as nutritional rehabilitation and support, or to assess the response of a patient to an intervention. The middle-upper arm circumference (MUAC) of an individual is an anthropometric measurement known to be a reliable indicator of skeletal protein and subcutaneous fat, both important determinants of malnutrition. In some studies, MUAC has been found to be better at predicting death in children than any other anthropometric indicator. The MUAC measurement, which is taken on the left arm, at the mid-point between the tip of the shoulder (i.e., the upper edge of the posterior border of the acromion process) and the tip of the elbow (i.e., the olecranon process), has been widely adopted as a favored anthropometric technique. MUAC measurement has been vigorously promoted as the preferred anthropometric technique by the World Health Organization (WHO), United Nations Children's Fund (UNICEF), and many governmental and private health organizations throughout the world. In addition to its value as a nutritional status assessment tool, MUAC measurement is favored because it requires no expensive equipment or extensive training, is portable, easy to use and widely applicable. MUAC measurement can be taken regardless of the age or health status of the individual or the ability to stand, sit or walk. MUAC measurements can be taken on even the most debilitated individuals.
Currently MUAC is widely used by international agencies to evaluate the nutritional status of children in medically underserved areas such as developing nations. More recently, The Academy of Nutrition and Dietetics and the American Society for Parenteral and Enteral Nutrition drafted a consensus statement with recommendations for diagnosing and documenting pediatric malnutrition in the United States, (Becker et al. J Acad. Nutr. Diet. 2014; 114: 1988-2000). MUAC was among the indicators of malnutrition the consensus panel recommended for evaluation. The panel proposed that “MUAC measurements should be part of the full anthropometric assessment in all patients, and are particularly important in those whose weight may be affected by lower-extremity edema, ascites, or steroids, as weight trends alone are unreliable related to fluid status. When serial z scores are unavailable, serial MUAC measurements can be used to monitor changes in body composition using the child as his or her own control. MUAC has been indicated as a more sensitive prognostic indicator for mortality than weight-for-height parameters in malnourished pediatric patients.” These entities advise that, “trained individuals consistently perform these measurements for best long-term comparison of data.” They further explicitly recommend that “z score, decline in z score, and negative z score” be used to classify and document pediatric malnutrition.
The World Health Organization has developed Child Growth Standards, comprising a set of population norms for malnutrition based on MUAC measurements for boys and girls from age 3 months to 5 years. While MUAC measurements can be taken with any measuring tape marked in millimeters, nutritional status is determined by referencing a set of norms that establish the measurements associated with various stages of malnutrition. Typically, these norms reflect a standard reference value that represents the central tendency (i.e., the mean or median) arm measurement for a nutritionally healthy child of a given age in the reference population. Variance from the reference mean value is measured in standard deviations. How far (i.e. how many standard deviations) away a child's measurement is from this population mean is represented by a number or standard score commonly known as the z-score. Thus, the reference mean value is assigned a z-score of 0. The various stages of malnutrition (specifically under nutrition) are defined by negative standard deviations from the mean, or negative z-scores. However, the World Health Organization (WHO) has developed a single child growth threshold for the identification of severe acute malnutrition in infants and children that is not based on z-score. It has issued the MUAC measuring tape that incorporates color bands which discriminate between severe acute malnutrition, moderate acute malnutrition, at risk for acute malnutrition, and normal in children up to 5 years of age. Importantly, the WHO MUAC tape reflects singular categorical color-banding thresholds for all children irrespective of age, such that reliability suffers considerably at the extremes of age.
While the use of z-scores in association with MUAC measurements has proven more reliable than the use of percentile deviations from a reference value, MUAC measurements for a population rarely produce a normal distribution represented by a bell-shaped curve. This is particularly true where there are instances of extreme differences in individual measurements within the population. For example, in some populations, the dataset of MUAC population values is skewed, or distributed asymmetrically, resulting in a curve that is shifted to the right or left of the reference value. Though skewness will influence z-scores, this is not accounted for if the MUAC data from the reference population are presumed to fit a normal standard distribution. Additionally, the peak of the distribution curve for the MUAC measurements in a reference population may be taller, sharper, or flatter than a standard bell curve. This so-called excess kurtosis occurs where more of the variability in the measurements is due to a few extreme differences from the reference value, rather than numerous modest differences. On the resulting curve, the probability of intermediate values is less likely and the central and extreme values are more likely. In settings of skewness or excessive kurtosis, the data must first be transformed to generate a near-normal distribution, then curve fit to define the mathematical function that best fits the data (traditionally a cubic spline), and finally back-transformed to arrive at the specific (i.e., lambda, mu, and sigma (“LMS”)) values from which the z-score can be calculated.
Currently available MUAC tapes endorsed by the WHO provide MUAC in millimeters and offer a single color scheme to indicate the risk for malnutrition. However, these tapes apply the same threshold values to all children irrespective of age. As discussed above, this singular threshold loses accuracy as children age. Consequently, application of the currently available MUAC tapes does not facilitate longitudinal assessments of children. Further, the currently available MUAC tapes provide no frame of reference for the distance from the mean that these children fall relative to children their own age. Additionally, the existing MUAC tapes do not take into consideration children over the age of 5 years, adolescents, and adults whose nutritional status may also be at risk. Finally, the existing MUAC tapes do not address malnutrition that contributes to overweight and obesity (overnutrition). Such individuals typically do not exhibit the loss of upper arm muscle mass associated with malnourishment and so are overlooked by current MUAC screening devices and methods. However, protein-calorie malnutrition is known to contribute to obesity, and such individuals may require nutrition support to address malnutrition. The current MUAC device does not provide a means to categorize such overweight and obese individuals according to their nutritional status and risk for malnutrition.
What is needed in the art is an improved device and method for estimating the nutritional status of an individual at any age, including an infant, child, adolescent, or adult without regard to variability in the dataset for the reference population. Further, what is needed is such an improved device and method that can be used in a population of all ages and across a broad weight range. Further, what is needed is such an improved device and method that can determine nutritional status (as defined by z-score) in a single step, without the need to identify the correct reference chart and determine a range from the chart. Further, what is needed is a method of making such a device based on creating a scale of numbers, indicia, or symbols that correspond to the circumference of the middle upper arm, middle upper thigh, chest or other circumference and its standard score for individuals of various ages and weights.